NI BTIS-mf—9024 fe LJOL aa/ruitLt IV ] of I '• I Il'•ir . GALAXIES IN LOW DENSITY REGIONS OF THE UNIVERSE I >A'ji Proefschrift ter verkrijging van de graad van Doctor in de Tl Wiskunde en Natuurwetenschappen aan de Rijksuni- r versiteit te Leiden, op gezag van de Rector f i Magnificus Dr. A.A.H. Kassenaar, hoogleraar in de faculteit der Geneeskunde, volgens besluit van het college van dekanen te verdedigen op woensdag 28 september 1983 te klokke 16.15 uur door NOAH BROSCH geboren te Boekarest (Roeaenie) in 1948 Promotoren-.Prof .Dr. J.liayo Greenberg W: Prof .Dr. W.W. Shane |r \f Referent :Dr.J. Lub / l I 'A '! 8.' ?•• i Table of Contents Chapter I Introduction and Background. .1 Chapter II Pccurate Optical Positions of Isolated Galaxies 10 1982,Rst ron.flst rophys .,Supp1. 46,63 Chapter III Photoelectric Photometry at the Wise 7; Observatory 17 Chapter Multiaperture Photometry of Isolated Galaxies 37 'i 1382,Rstrophys.J. 253,526 Chapter V Multiaperture Photometry of Galaxies:II. Near Infrared Photometry of Six Isolated i Objects 50 1982,Rstron.Pstrophys. 113,231 :J 't" Chapter VI 5 GHz Observations of Isolated and Group Galaxies 56 1983.Submitted to flstron.Rstrophys. Appendix R Samples of Isolated Systems 85 Chapter VII Ultraviolet Spectrophotometry of Five Isolated Galaxies 90 1383,Submitted to Rstron.Rstrophys. Appendix B Isolated and Normal Galaxies in the UV...113 Chapter VIII Rdditional Observational Data and Overview of the Thesis ,. 117 Samenvatting 134 t Curriculum 136 acknowledgements ' 137 Hebrew Summary 138 Chapter I p V Kant speculated in 1755 that the nebulae,now called >:c: galaxies,were remote, independent stellar systems, or "island |' universes',as they were subsequently called (Shane,1975). Since by' 1755 our ideas about the structure of the Universe and the ' distribution of the galaxies therein have advanced tremendously. [;:-- In particular the true three-dimensional structure of the .?'/ Universe in the immediate neighborhood of our galaxy ( to beyond - 'U the Virgo supercluster ) is being unraveled these days by •%•'$ extensive redshift surveys. '•'' ?•-'j The distribution of galaxies over the sky is highly non- : y 'i ' uniform, fis far back as the beginning of this century Wolf (1906) identified concentrations of nebulae. Later on.different catalogs trr• .: n Holmberof clusteringg (1950,1969s of galaxie} . Howevers were, publishethe twdo maiby nShaple catalogy (1933s no)w anind ;'• •' current use ar* the Catalog of Galaxies and Clusters of Galaxies ;. f'. (Zwicky ct al.,1961-1968) and the Catalog of Rich Clusters of £--V Galaxies CRbell.1958). ' . The way of selecting clusters of galaxies for inclusion in both v -; catalogs was by observing an enhancement of the surface number ":'i density relative to the density of the background. Inclusion in .;.; v Zwicky's catalog implies simultaneously that : the detected £y' cluster contains at least 50 galaxies whose magnitudes are within :ï| 3 mag of the brightest member of the cluster,all the cluster ||:! galaxies lie within a contour defining an enhancement of twice *; the neighboring projected density end the cluster lies north of \: declination -3*. Obeli's criteria for including a cluster in his 5V lists are stricter : the cluster must contain at least 50 members -2- brighter than nij +2,where m3 is the magnitude of the third brightest member galaxy,all within a circle with radius 1.7/z arcmin (z is the redshift of the cluster,estimated by Pbell from the magnitude of the 10-th brightest galaxy in the cluster);the redshift of an Hbell cluster has to be 0.02 < z < 0.20 and the cluster should lie north of declination -27°. Not only clusters of galaxies were catalogued. Lists of groups of galaxies ,apparently characterized by a lower population,were produced by Holmberg (1937) and later by Sandage and Tanmann (1375),de Vaucouleurs (1375) and Turner & Gott (1376a). The definition of a group of galaxies in these works is similar to that of a cluster of galaxies,i.e. a projected density enhancement. For example,the Turner and Gott groups have to be i north of declination 0*,north of galactic latitude 40° and only galaxies brighter than 14.0 mag have been considered for inclusion. The density enhancement considered by Turner and Gott is a factor of 4.64 * dex(2/3) over the average projected density. It is clear that galaxies are gregarious entities which tend ':,; to clump in the Universe. This tendency was apparently noted £j already by William and Sir John Herschel (cf. Materne and [I Tamman,1974). This clustering is most evident in the extensive 'ii survey of galaxies conducted at the Lick Observatory (Shane and j[j ,: Wirtanen,1967). The study of the galactic neighborhood,of all | I,: galaxies known within 9.1 Hpc showed that 2/3 of all the objects | 'd. p » within this volume are concentrated to only 7 groups (Tammann and [| '4'r Kraan,1977). In spite of this,only a small fraction of galaxies A YA resides in the rich clusters,while more than 90 % are in many ': low-density loose groups,or are isolated (Dressler, 1980a). R large amount of observational and theoretical effort has I -3. ••• • been expended to try and understand galaxies in clusters. This is -; understandable,because the cluster environment enhances the sj~ chance of interactions between galaxies. The probability of one '" galaxy encountering a similar one is proportional to its '" geometrical cross-section TTr1, its peculiar space velocity v.the space density of galaxies N,and the time passed t. In other kf words: P~**r**v*N*t (1) :i ; The observed quantities are <v>,Nh~ and rh,where h is the Hubble constant in units of 100 km/s/Hpc ,thus P~7*HDSCrh/10a kpcJ*t<v>/100 km/s3*CNh"*/1 Mpc*J*h«r (2) T is here the time passed between subsequent collisions and ••, ;t encounters in units of the Hubble time. •'"' In a rich,regular type cluster of galaxies the central density r is (BahcaU,1977> .4*10'l«ha galaxies/Hpcs (35 The average velocity of a galaxy is also higher than in poorer V: clusterings,of order 10 tcmtcm//s . There,the timescate between close \\ encounters is (B*hcall,1977) r. ~ - yrs (4) 1 <v> / (1000km/s)«N,/(1000/Mpcs)*rh/10kpc In such environments close passages within 100 kpc will occur on average every 10 yrs (h*1),much shorter than the Hubble time. Rs a consequence,in high-density environments the galaxies probably suffer an appreciable number of collisions and close encounters. Consider the situation in a typical group of galaxies,in contrast to that in a rich-cluster environment. The relevant -4- '• figures are then <v>~300 km/s and N~7 gal /Mpc (Turner and Gott , ;.. 1976b),formally implying E~5 10 yrs. a very long time indeed. '.'•' r Rn even longer collision timescale is obtained considering field i\, galaxies which are more or less isolated ,and for whom <v>~70 •- km/s (Rivolo and Yahil,1981;Tsvttkov,1981) and N~2 gal/Mpc3(this \ thesis). Probably the lowest collision timescale is that for i'v T galaxies in cosmic voids. There N can be as low as 0.0006 gal/Mpc* ' (Perseus/Pisces void;Doroshkevich,5handarin and Zeldowich, 1982) ,while the velocity dispersion,though unknown,is believed to be small. I How does the general density of different regions affect the i nature of the luminous matter therein ? Dressier (1980b) has shown that regions of low density are populated mainly by spirals and by irregular galaxies,while in the environments with the highest density these morphological types are rare. This was :'t known qualitatively long before . Spitzer and Baade (1951) attributed the paucity of spiral and irregular galaxies in rich r> clusters to collisions among the cluster galaxies which resulted in the stripping of these objects of their interstellar gas '"' content leaving them gas-poor. Rn even more spectacular result of collisions could be the ring galaxies ; one explanation for their formation put forward by Freeman and de Uaucouleurs (1974) is that these peculiar objects are born out of collisions between spiral galaxies and clouds of gas similar in size to galactic systems. But grazing tidal encounters ,which have larger impact $ parameters«are far more common than direct collisions (see [i equation 2 above ftr an estimate of the probability) and produce fe» peculiar morphological configurations, like bridges and tails '/ÏÏ1f (Toomre,1972 and many others since). That such encounters may -s- modify the mass distribution within a galaxy,was shown by dynamical N-body simulations (Roos and Norman, 1979,-Dekel et al.,1980). Even galaxies in isolated binary systems may interact with each other. It is widely believed that isolated spiral galaxies are formed with extensive massive halos. In pairs of galaxies the halos will be the components most sensitive to interaction. Tidal forces between the members of the pair may strip off halo material ; a common envelope may form or some material may be lost altogether from the system. Loss of material,which carries off energy and angular momentum,implies evolution of the binary orbits,thus mergers of galaxies are possible. Not only galaxy-galaxy interactions can affect the properties of the observed objects. The medium permeating at least some clusters of galaxies is apparently filled with very hot gas (T~10*K .Mushotzky ct al.,1976). The intra-clustcr medium (ICM) can change the observed properties of the cluster galaxies via at least three different processes. If the subject galaxy is moving rapidly through the ICM , due to its being in virial equilibrium 3| in the gravitational potential well of the cluster,effects of ram pressure can cause stripping of gas from the (disk) galaxy (Gunn and 6ott,1972).